The present invention relates, generally, to a self-contained machine for polishing, cleaning, rinsing, and spin-drying semiconductor wafer workpieces and, more particularly, to an improved system for receiving a cassette of wafers; Chemical Mechanical Polishing (CMP), cleaning, rinsing and drying the wafers; and returning the polished and cleaned wafers to the same cassette and to the same slot from which they were taken.
Machines for polishing and machines for cleaning wafers and disks in the electronics industry are generally well known. For example, semiconductor wafers, magnetic disks, and other workpieces often come in the form of flat, substantially planar, circular disks. In the manufacture of integrated circuits, semiconductor wafer disks are sliced from a silicon ingot and prepared for further processing. After each wafer is sliced from the ingot, it must be thoroughly polished and then cleaned, rinsed, and dried to remove debris from the surface of the wafer. Thereafter, a series of steps are performed on the wafer to build the integrated circuits on the wafer surface, including applying a layer of microelectronic structures and thereafter applying a dielectric layer. Typically, after the layers are fabricated on the wafer surfaces, the wafers must be planarized to remove excess material and imperfections.
After each processing step, it is often desirable to thoroughly clean, rinse, and dry the wafers to ensure that debris is removed from the wafers. Thus, a method and apparatus for quickly and efficiently cleaning, rinsing, and drying wafers is needed which facilitates high wafer throughput, while at the same time thoroughly cleaning and drying the wafers with a minimum of wafer breakage. For a discussion of existing wafer cleaning machines, see, for example, Lutz, U.S. Pat. No. 5,442,828, issued Aug. 22, 1995; Frank et al., U.S. Pat. No. 5,213,451, issued May 25, 1993; and Onodera, U.S. Pat. No. 5,357,645, issued Oct. 25, 1994.
Presently, CMP polishing and/or planarization is performed by one machine and wafer cleaning and drying is performed by another, separate machine. After a processing layer (i.e., oxide, tungsten or the like) has been deposited on the surface of the wafers, the dry wafers are placed in a cassette and hand carried to a CMP polishing machine. The CMP machine removes excess material by planarizing the wafers, and then typically rinses the wafers and places the wafers into a wet cassette. After polishing, residual particles still reside on the wafer""s surface. If these particles dry on the wafer prior to cleaning, the microelectronic structures on the wafer may be corrupted. Therefore, it is extremely important to keep the wafers wet prior to cleaning and drying the wafers. From the CMP machine, the wet cassettes are hand carried to a separate wafer cleaning and drying machine which is typically located somewhere near the CMP machine.
This conventional practice of utilizing separate machines for wafer polishing and for wafer cleaning and drying has serious drawbacks. First, wafer manufacturers must have personnel, equipment and facilities on hand to transport wafers in a wet environment from a CMP machine to a cleaning and drying machine. Secondly, having separate machines for polishing wafers and for cleaning wafers consumes a significant amount of clean room space which, as one skilled in the art will appreciate, is very expensive.
The present invention overcomes the shortcomings of conventional prior art systems by integrating the polishing, cleaning and drying functions in one machine.
Accordingly, a primary object of the present invention is to provide a combined wafer CMP polishing, cleaning and drying machine in which wafers are removed from a dry cassette, polished, cleaned, dried and returned to the same cassette and slot from which they were removed.
Another object of the present invention is to maintain the load station, the cleaning stations and the drying stations of the combined polishing and cleaning machine at a class 1-10 clean room environment. Positive laminar air flow from the load and cleaning stations of the machine into the CMP station of the machine is utilized to maintain the clean environment within the load and cleaning stations. The positive air flow ensures that slurry and other particles that may be liberated from the wafers during polishing do not migrate into or otherwise contaminate the clean environment.
Another aspect of the present invention is utilization of a six-axis robot to remove wafers from a dry cassette and to transport the wafers to an index table within the CMP station of the machine. The robot is also configured to transport wet wafers from a rinse station in the cleaning station of the machine to a spin dryer station also located in the cleaning station. The robot also removes the dry wafers from the spin dryer station and places them back into the cassettes located in the load station of the machine. The robot has a wet end effector and a dry end effector to ensure that wet and dry wafers are isolated during transport. The dry end effector is used to unload and load dry cassettes, and the wet end effector is used to move wet wafers from the rinse station to the spin dryer station.
Another feature of the present invention is a wafer mapping system which determines which slots within a wafer cassette is occupied by wafers. The mapping system also determines whether wafers are properly aligned within the slots and whether more than one wafer is within a particular slot. The mapping system preferably comprises an optical scanning device, such as a video camera, mounted in a mounting bracket attached to a top portion of the robot and a system processor configured to interpret and process the signals from the scanning device. When a cassette is placed on the polishing and cleaning machine, the robot end-effector retrieves the scanning device (camera) from the mounting bracket and traverses up and down in front of the cassette, allowing the optical scanner to view the contents of the cassette. Additionally, a back lighting source behind the cassette may be utilized to increase the effectiveness of the optical scan vision system.
The CMP station is preferably configured to receive and polish five wafers at a time. After the wafers are loaded by the robot onto an index table, a multi-head transport apparatus lowers five wafer carrier heads into proximity with the index table and picks up the wafers. The transport apparatus then moves laterally until it is positioned above a polishing surface. The transport apparatus is then lowered such that the wafers are pressed against the polishing surface. To enhance the polishing process, a polishing slurry is preferably provided, and the individual carriers are rotated on and oscillated radially across the polishing surface. After polishing, the wafers are returned to unload cups in the index table. A flipper apparatus then transfers the wafer from the unload cups to the cleaning station of the machine.
The cleaning station of the machine preferably comprises a water track, cleaning stations, a rinse station, a spin dryer station and a plurality of wafer staging areas. More specifically, when a wafer is first loaded into the cleaning station from the CMP station of the machine, the wafer is held at a first staging area until the machine determines that it is clear to release the wafer. When cleared, water jets urge the wafer into a first cleaning station configured to wash and clean both surfaces of the wafer. From the first cleaning station, the wafer is transported down a water track into a second staging area. Again, the wafer is held at this position until the machine determines that the wafer in front of it has cleared to the next station. From the second staging area, water jets urge the wafer into a second cleaning station for a second cleaning of the wafer. The wafer then exits the second cleaning station into a third staging area. From the third staging area, the wafer is transported down a water track to the rinse station. After rinsing, the robot moves the wafer to the spin dryer station, and then to a cassette.
The wafer cleaning stations preferably comprise a plurality of pairs of rollers which pull the wafers through the cleaning stations and which also clean the top and bottom flat surfaces of the wafers. Various rollers within the roller boxes may operate at different rotational speeds and rotate in different directions. In this manner, certain rollers may function as drive rollers to move wafers through the cleaning stations, while other rollers may function to clean wafer surfaces as the wafers are driven through the cleaning stations.
In a particularly preferred embodiment, the rollers are contained in enclosed boxes which may be easily removed from the machine to facilitate convenient changing of the rollers as the roller surfaces become worn through extended use. A plurality of channels are preferably formed in an upper inside surface of the roller boxes to permit distribution of a plurality of different chemicals (e.g., water, cleaning solutions, surfactants, friction reducing agents, and agents to control the pH of the various solutions) into discrete regions of the roller boxes. In this manner, wafers passing through a first set of rollers may be exposed to a first chemical solution and later exposed to a second chemical solution in a latter stage of the roller box. Since a plurality of roller boxes are preferably employed, different chemicals may be used in different cleaning stations. The first roller box, for example, may distribute a cleaning solution and deionized water mix onto the wafers to facilitate heavier cleaning, while the second roller box may simply distribute deionized water onto the wafers to achieve a rinse.
Wafers are transported from the second cleaning station to the rinse station via a water track. The water track is supported by support posts and the support posts may incorporate a vertical adjustment system for track alignment. Wafers are rinsed in a serial manner within the rinse station, which is configured to tilt downward during the rinsing procedure. The downward tilt facilitates effective drainage and removal of any debris or chemicals. A number of water jets urge each wafer into the rinse station, maintain the position of the wafer during rinsing, and perform the rinsing of the upper and lower surfaces of the wafer. The water jets also support the wafer within the rinse station such that mechanical contact with the wafer is minimized.
After rinsing, the robot uses a wet end effector to lift wafers from the rinse station and transfer them to a spin dryer station. The spin dryer station is equipped with a motor which spins a platform holding a wafer at speeds in the range of about 4,000 rpm, to thereby remove residual deionized water from the wafer. The spin dryer station preferably includes a shield around the spinning apparatus to guard other stations of the machine from water and debris that may be shed during the spin-dry process. The shield preferably includes a movable door so that the robot can access the spin dryer. The spin dryer preferably employs a number of gripping fingers configured to maintain the wafer upon the platform during spinning.
The robot uses a dry end effector to retrieve dry wafers from the spin dryer station and to return the dry wafers to the cassette from which they originated. Each wafer is tracked and monitored through the polishing and cleaning steps so that after processing, it can be placed back into its original slot in its original cassette.
A vision system or other position sensing method may be utilized to monitor wafers as they pass through the cleaning station of the machine and to determine if the wafers have properly moved from one area of the cleaning station to the next. Wafers are released from the various staging areas when it is determined that the wafers are all safely positioned within the proper staging areas; that is, when it has determined that lodged wafers are not still in the water track or in the various cleaning, rinsing and drying stations.
The flow of fluids to the wafer index station, polish station and cleaning station may be controlled through use of a fluid flow regulator system, which monitors the flow of fluid, as opposed to prior art systems which typically measure fluid pressure. By measuring fluid flow directly, the system is less susceptible to variations in inlet fluid pressure. Fluid flows within the system may thus be much more accurately controlled than is possible with prior art systems.
The present invention may also incorporate an operator interface such as a flat panel touch screen. The touch screen preferably presents a three-dimensional graphical image of virtually every relevant aspect of the system to facilitate operation, maintenance, trouble-shooting, and the like.